Optical materials include substances whose function is to alter or control electromagnetic radiation in the ultraviolet, visible, or infrared spectral regions. Optical materials are fabricated into optical elements such as lenses, mirrors, windows, prisms, polarizers, detectors, and modulators. These materials serve to refract, reflect, transmit, disperse, polarize, detect, and transform light, including light in the visible ultraviolet and infrared spectral regions. Atoms and their electronic configurations in optical materials interact with electromagnetic radiation to determine the material's macroscopic optical properties such as transmission and refraction. These optical properties are functions of the wavelength of the incident light, the temperature of the material, the applied pressure on the material, and in certain instances the external electric and magnetic fields applied to the material.
Most optical elements are fabricated from glass, crystalline materials, polymers, or plastic materials. In the choice of a material, the most important properties are often the degree of transparency and the refractive index. The uniformity of the material, the strength and hardness and temperature limits may also need to be considered. Optical materials are used in a wide variety of applications, including photolithography and medical devices such as endoscopes.
Medical instruments utilizing optical components are delicate and may easily break or become damaged by handling or by fluid contamination. Independent service companies in the field do not have access to replacement parts. It is necessary to reverse engineer these parts by determining all physical dimensions and materials. In many fields, such as with medical devices, optical components are not optimized as an individual component. Instead, the entire optical system is optimized as a complete unit. For this reason, the dimensions and materials of the replacement parts must exactly replicate the original parts. The performance of the optical system may be compromised if the replacement components are not identical to the manufacturer's components.
Optical materials are characterized by their refractive indices at well-defined wavelengths. Refractive Index (RI) is a function of the composition and thermal history of the material. The RI of a substance (i.e., an optical medium) is a number which describes how light, or any other radiation, propagates through that medium. The RI is mathematically expressed as ni=v1/v2, where refractive index=n1 at a specific wavelength i, and the speed of light in each media are v1 and v2. For glass analysis, v1 is the speed of light in a vacuum or air.
Optical materials are also identified by the approximating the dispersion of the substance. Dispersion is the change in refractive index with a change in wavelength of illumination. All optical materials share the same typical dispersion curvature. As a result, optical materials are commonly characterized by the refractive indices of only three defined wave lengths. A first wavelength is selected from the yellow/green region of the visible spectrum. The two remaining wavelengths are selected from the blue and red region of the edge of the visible spectrum. Commonly referred to as V, relative dispersion is a measurement of the difference between the refractive index at different wavelengths of light, typically nC (486 nm), nD (589 nm), and nF (656 nm), mathematically expressed as V=(nD−1)/(nF−nC) (commonly referred to as the “Abbe number). Several mathematical functions approximate the refractive index as a function of the wavelength, referred to as a dispersion formula. Exemplary dispersion formulas include the Schott formula, the Sellmeier formula, the Herzberger formula and the Hartmann formula. One of the most commonly used formulas utilized with high precision optical glass is the Sellmeier formula.
The Becke line method is a method for determining the refractive index of a transparent particle relative to its surrounding medium. A Becke line is the bright halo near the edge of a transparent particle immersed in a medium. The halo moves with respect to that edge as the focal plane of the microscope is changed. The RI may be measured by noting the direction that the Becke line moves when the distance between the objective of the microscope and the preparation is changed. The Becke line will always move toward the higher refractive index medium when the distance is increased and will move toward the lower refractive index medium when the distance is decreased from the point of critical focus. The Becke Line Method can't provide a quantitative determination of the differences between the two RIs. Additionally, two materials with nearly equal RI values will have bright boarders with a rainbow color, making a determination of the relative RI values impossible.
A second method of identifying optical materials utilizes a phase contrast microscope to differentiate between two materials where one is embedded in the other. Light will pass through the materials at different speeds if the materials have different refractive indices. Light passes through the materials at the same speed if they have the same RI. This is evidenced by the disappearance of the contrast between the two materials.
A method of comparing optical materials is termed the “Emmons Double Variation Method”, which relies on the relationship between RI, temperature and the wavelength of light. A grain of an optical material is embedded in a liquid with a similar RI. The differing shapes of the diffusion curves of the two materials cause their RIs to match at only one wavelength, where the phase contrast will vanish. This wavelength is referred to as the matching point. The matching points in the Emmons method are determined at varying temperatures or by changing the wavelength of light. The RI of the immersed material can be determined by varying the temperature, causing a change in the RI curve of the immersion media. It is often difficult to efficiently transfer heat from the heating chamber to the sample, thus making this method laborious and time consuming.
What is needed in the art, therefore is a fast, reliable and documented procedure for identifying optical materials. Such a technique should distinguish optical materials with a precision necessary to reproduce compounds with such materials so that they may be used in medical instruments.